BACK CARE FOR SITTING WORK

Sitting has become a way of life for many Americans. We sit in cars, buses, trains or planes when we travel, we sit to eat meals, we sit and watch TV, we sit in classrooms, and for many of us, we sit most of the day at work, often in front of a computer all day. Studies of sedentary workers show that low back problems are associated with poor chair design and inappropriate sitting posture.

Although for many activities we could stand just as easily as sit, for example, we could stand to watch TV, stand to eat, and stand in front of a computer all day. There are at least three good reasons why we prefer sitting.

1. Sitting uses about 20% less energy compared with standing to do the same work, so comfortable sitting helps to relieve fatigue.
2. Sitting helps to reduce the strain on our back muscles and on the intervertebral discs of the lumbar spine, providing that we sit back in a supported, reclined posture.
3. Sitting gives us greater postural stability for performing fine manipulative tasks, such as eating with a knife and fork, sewing, writing, etc.

Anatomical Changes during Sitting
When we change from a standing posture to a sitting posture anatomical changes occur. The lumbar spine changes shape depending on what we sit on and how we sit. If we sit on a flat surface, such as a bench, bleachers or stool, without any back support we tend to hunch the body forwards for support, often resting our arms on our legs to reduce fatigue. As we hunch forwards, the lower back curves outwards into a kyphotic shape. This is generally regarded as an unhealthy posture if sustained for a prolonged period. With nothing to lean back on, the upper body becomes fatigued and we typically regard this as an uncomfortable way to sit. However, this is the way that many people end up sitting over the course of each workday, so it isn't surprising that studies of sedentary workers, such as office workers, have frequently reported high levels of postural discomfort. [2] What we want is to be able to sit and maintain the lumbar spine in a posture called lordosis.  

Ergonomic Chair Design
If we sit on a good ergonomic chair, the seat pan is curved from back to front to encourage the pelvis to rotate forwards, and this helps the lumbar spine to maintain lordosis. As we sit back the lower back should be well supported by a contoured lumbar support. The average preferred height for a good lumbar support is abut 7.5 -inches above the compressed seat surface (19 cm), and the mean preferred seat depth (horizontal distance from front of seat to lumbar support point) is about 15.25-inches (38.7 cm). Ideally, the chair should have a backrest that is sufficient high to provide support to the thoracic region as well. [3] Also, make sure that the seat is height adjustable (15" - 22"), the backrest angle is adjustable (100° - 120°), the armrests have a minimum 16.1” width and have adjustable height (7.1”-10.6” above compressed seat height). [4] Studies of office chairs show that users often cannot correctly identify or operate chair controls [5], so make sure that the chair has user-friendly controls.

Safe Sitting
Keeping the spine healthy requires periodic changes in posture, and dynamic movement helps to promote circulation and reduce muscle fatigue. Sitting in any static posture for a prolonged period eventually will become uncomfortable. This means that there isn't a fixed posture that's best for everyone, all of the time, whatever the task at hand. Rather, there is a desirable range of movement that works well for most people doing most of the kinds of tasks performed when sitting.

The preferred way of sitting involves the following:

• Make sure that the seat height is correctly adjusted so that your feet are on the ground or on a solid surface like a good footrest.
• Recline back in the chair, with the chair backrest angled between 100-110-degrees, so that the chair back can help support the weight of the torso.
• Make sure that the chair has good lumbar support in the right area for your shape and size of back. If there is an adjustable support, use this to get the best position. If not, use a rolled towel or a cushion to improve your lower back support.
• Make sure that the seat pan is the right size for you and doesn't press behind your knees.
• Look for a chair that doesn't tip up the seat pan when you recline because this can put pressure under the thighs and behind the knees.
• If the chair has arms, make sure that these are correctly adjusted for height so that your shoulders are relaxed, not hunched or raised when you rest on the armrests.
• If the chair has a high neck/headrest make sure that this can be used in different sitting positions.

Now that your body is in a good, supported posture you should be ready to work without any discomfort, but remember to take periodic breaks, and to change posture, get up and move around.

Professor Alan Hedge, PhD
Director Human Factors and Ergonomics Laboratory
Cornell University
Department of Design and Environmental Analysis
Ithaca, NY